In a small saucepan, combine flour and sugar.,Whisk the egg and the milk, stir into flour mixture until smooth.,Bring to a boil over medium heat; cook and stir for 1-2 minutes or until thickened and bubbly.,______,Just before serving, combine the apples and celery in a large salad bowl.,Drizzle with dressing; gently toss to coat.,Sprinkle with grapes and walnuts.